The present invention relates to an underwater trenching apparatus and more particularly to a self propelled trenching apparatus for movement along a pipe lying on the bottom of a body of water to excavate a trench under the pipe.
Such trenching machines are shown in U.S. Pat. Nos. 3,429,131; 3,429,132; 3,717,003; 4,022,028; and 4,274,760. Typical of the background problems associated with such equipment are the devices shown in U.S. Pat. Nos. 3,717,003 and 4,274,760.
Bates Jr. et al. patent (U.S. Pat. No. 3,717,003) shows a trenching apparatus for use in operations similar to that of the present invention wherein a pair of rotatable cutter heads is supported by a carriage which is positioned on a pipeline to be buried. The cutters are arranged to dig a trench beneath the pipeline over which the carriage moves, whereby upon removal of the cuttings from the trench, the trailing portion of the pipeline is free to fall into the trench. A suction conduit is positioned adjacent to the cutters to suction cuttings from the vicinity of the trench. Two separate driving wheels are positioned on top of the pipe and an idler wheel is positioned beneath the pipe and urged upwardly against the pipe by a yieldable force which clamps the pipeline between it and the upper wheels. Both upper and lower wheels are positioned behind the trench cutters and behind the longitudinal center of the carriage. Fore and aft buoyancy tanks are used to reduce the load or weight of the apparatus on the pipeline. One set of rollers having four individual rollers mounted on top, below and on each side of the pipeline limit movement of the carriage in any direction laterally with respect to the pipeline. These rollers are not urged into gripping contact with the pipeline. The arrangement described above is not rotationally stable about the pipeline for a number of reasons. For example, the wheels driving the apparatus along the pipeline are positioned behind the center of the carriage tending to facilitate a longitudinal tilt of the carriage. Also, there is not sufficient control of the buoyancy tanks to balance the load of the carriage on the pipeline under varying conditions of water depth, pipeline diameter variations, tilt of the pipeline itself, etc. These conditions and others permit the carriage to lift from the pipeline and thus drive off laterally from the pipeline. This in turn causes the carriage to rotate about the longitudinal axis of the pipeline.
Norman patent (U.S. Pat. No. 4,274,760) shows a trenching device having a roller driving system designed to avoid the use of divers in moving the apparatus past obstructions on the pipeline, such as anodic collars positioned about the pipeline at spaced locations. Norman shows the use of fore and aft driving side rollers which are urged into contact with the pipeline but which are selectively disengageable in pairs from contact with the pipeline to permit passage of the rollers over an obstruction. Upon passage of the leading pair of rollers over an obstruction, the leading pair of rollers are closed on the pipe and the trailing pair of rollers are opened from engagement with the pipe until their passage past the obstruction whereupon the trailing pair of rollers are re-engaged with the pipe. The above-described operation is cumbersome and time consuming. The frame structure of the Norman device is constructed so that the rollers, when pushed outwardly from the pipeline, pivot radially about the top of the carriage, bending the carriage frame longitudinally. The arrangement of supports for the rollers including the disengaging mechanism provides a parallelogram structure which misaligns itself on the pipeline under certain operating conditions. In addition, control of buoyancy tanks is limited so that variations in water depth as well as other variable factors, including roller disengagement and eccentric forces resulting therefrom cause the carriage to shift on the pipeline to the detriment of the trenching operation.
Additionally, none of the above-referenced patents show an apparatus which can be moved back and forth along the pipeline to facilitate variable depth entrenchment.
It is therefore an object of the present invention to provide a new and improved trenching system for stable movement and weight control on a pipeline to facilitate accurate and trouble-free operations and a system which can be operated in either direction to facilitate variable depth entrenchment.